This invention relates to an apparatus which electronically indicates an abnormal condition in a rotatable member or plurality of rotatable members. The invention has particular applicability in detecting a low pressure condition in the tires of a vehicle while in operation.
Many devices which include rotating members employ apparatus to monitor the rotating member to alert the operator of an abnormal condition. For example, in certain machinery it is highly desirable to maintain the speed of rotating members within close tolerances. Usually, governors, regulators, synchronous speed control devices or the like are used for such purposes. But these devices can be expensive and are not universally adaptable for all applications.
Another abnormal condition in a rotatable member which should be monitored includes the pressure of an inflatable member such as a pneumatic vehicle tire. An undetected low pressure condition in a tire is not only a hazard to the safe operation of the vehicle but also is detrimental to the tire itself. Such low pressure may go undetected for hours in situations involving long trips or the like.
A number of electronic devices have been employed to detect such a low pressure condition, but all have their drawbacks. For example, in one type of device, a low pressure condition is sensed by counting tire revolutions and comparing such revolutions with the revolutions of a properly inflated tire to determine if one tire has a smaller diameter than the properly inflated tire which would be indicative of a low pressure condition. However, in the increasingly popular radial tire, a tire where pressure tolerance is highly important, there is little deflection under a low pressure condition such that this method would be relatively unworkable. Similarly, in the newly developed cast tire there is very little deflection change related to pressure.
Numerous other devices use radio circuits to warn of a low pressure. These devices usually require power sources at each wheel to operate a transmitter with the receiver being in the cab of the vehicle. When a low pressure condition is detected, the transmitter is activated to warn the operator. Such operation is highly susceptible to false alarms from other transmitters or system failure due to the fact that the electromagnetic waves can be blocked by the metallic portions of the vehicle body, particularly in the situation involving a tractortrailer.
More recently ultrasonic type devices have been used to indicate a low pressure condition. But again these devices are susceptible to spurious activation. In short, none of these devices are failsafe, fully testable, and substantially free of maintenance problems.